According to French media, the attacker served several years in prison for firing on police officers with a gun in the early 2000s.

More recently the intelligence services identified him as a potential Islamist radical.

Meanwhile, IS named the attacker as Abu-Yusuf al-Baljiki, in a statement carried by its Amaq news outlet.

The Belgian interior minister told VRT public broadcaster that the perpetrator was a French national.

Could the attack influence the elections?

The attack took place as 11 candidates in Sunday's closely fought presidential election were engaged in a final joint TV appearance to argue their policies.

Three of the four main candidates, centrist Emmanuel Macron, centre-right Francois Fillon and far-right Marine Le Pen, have called off planned events on Friday, which would have been the final day of campaigning.

Front National candidate Marine Le Pen told a French radio station on Friday morning she feared further attacks, and said France should immediately reinstate border checks.

She tweeted: "I feel for and stand by our security forces, who have been targeted again."

Meanwhile, Independent centrist Emmanuel Macron urged French citizens not to "give in to fear" in a interview with the RTL station.

He said it was a president's "first duty to protect" and he expressed his "solidarity" with the police.

What is known about the attacker?

Paris prosecutor François Molins said shortly after the shootings that "the attacker's identity is known and has been verified".

"I won't reveal it, because investigations and raids are already under way, in particular to establish whether there is any evidence or not of complicity [in this attack]," he said, adding that more information would be released on Friday.

Mr Fillon, of the Republican Party, also went on Twitter to pay "tribute to the security forces who give their lives to protect ours".

Jean-Luc Mélenchon, standing for the far left, tweeted: "I strongly feel for the policemen killed and wounded and their families. Terrorist attacks will never go unpunished, accomplices never forgotten."

Analysis by BBC's Europe editor Katya Adler

It would easy to assume that Marine Le Pen - so outspoken about security, migration and Islamic fundamentalism - could benefit at the ballot box.

But anxious voters may turn, instead, to experienced conservative politician and former Prime Minister François Fillon.

Image copyrightREUTERS
Image captionPeople could be seen walking towards police with arms raised to show their hands after the shooting

Up until now, four candidates were almost neck-and-neck in the lead in the polls with millions of voters still undecided about who to vote for.

It's hard to tell to what extent this attack will affect the outcome, but centrist Emmanuel Macron and Jean-Luc Mélenchon on the far left will be all too aware that few see them as foreign and security policy heavyweights.

Islamist militancy is a major issue in the polls after recent mass attacks claimed by IS, with 238 people killed in jihadist attacks in France since 2015, according to data from AFP news agency.

And how did the world react?

At the White House, US President Donald Trump said people had to be strong and vigilant.

"Our condolences from our country to the people of France," he said. "It looks like another terrorist attack and... what can you say? It just never ends."

In the UK, a Downing Street spokesman said: "The UK strongly condemns the appalling terrorist attack in Paris. The Prime Minister (Theresa May) has tonight passed on her condolences to President Hollande."